In order to minimize fuel losses to the atmosphere by evaporation from the gasoline tank and other liquid fuel sources associated with an automobile engine, the fuel vapors are commonly vented to a charcoal canister for adsorption therein when the engine is not operating, and are vented from the canister to the engine fuel-air intake manifold when the engine is operating. In order to prevent entrainment of liquid fuel from the tank with the fuel vapors being vented to the canister, as for example when the automobile is driven along a steep incline or as a result of splashing when the automobile is driven over rough roads, the vent line customarily communicates with the fuel tank at an upper region thereof above the normal fuel level. In addition various types of vapor separators are employed to cooperate with the opening of the vent line or conduit into the fuel tank.
The vent line is customarily provided with an overflow limiting valve comprising a low pressure one-way check valve adapted to open when the pressure in the tank rises to about 8 or 10 inches of water above atomosphere. Accordingly after the tank is filled at the filling station pump to the level of the filler spout opening into the tank, the vapor pressure above the fuel level will increase rapidly as more fuel is added, causing the auomatic shut-off valve for the pump nozzle to be triggered before the overflow limiting valve opens, thereby preventing accidental overfilling of the tank. A small bleed orifice by-passes the overflow limiting valve to prevent collapse of the tank as fuel is consumed or in the event of a large drop in fuel tank vapor pressure.
When the automobile is driven on a hot day, the additional heat from the customary exhaust pipe that passes in proximity to the fuel tank causes such rapid fuel evaporation within the tank that the venting capacity of the vent line and overflow limiting valve is exceeded and an excessive fuel vapor pressure in the tank results, often amounting to more than 50 inches of water. In such an event, the pressure vented closure cap for the tank serves as a safety valve and vents fuel to the atmosphere to prevent damage to the fuel tank. However, venting to the atmosphere is obviously wasteful and a source of air pollution.
It is undesirable to increase the venting capacity of the overflow limiting valve to accommodate such emergency situations when the engine is operating because then the overflow limiting valve would also vent fuel vapors faster than could be adsorbed by the charcoal canister during frequently occurring high temperature summer time conditions when the engine is not operating. Accordingly, the venting capacity for the overflow limiting valve must be limited to the extent that it will restrict the vent flow to the charcoal canister to a predetermined maximum rate that can be effectively adsorbed by the canister, so as to avoid overloading the latter and discharging the fuel vapor to the atmosphere when the engine is not operating. In consequence the overflow limiting valve will effect a moderately high but safe vapor pressure within the fuel tank, which in turn will retard the rate of fuel evaporation and enable the use of a charcoal canister of convenient capacity.
An object of the present invention is to provide an improved and highly efficient automobile fuel tank vent means to the charcoal canister for maintaining the desired vapor pressure in the tank during conditions of normally to be expected high ambient temperatures when the engine is not operating, and for increasing the venting capacity to the extent necessary to prevent damage to the tank when the engine is operating at unusually high ambient temperatures.
Another object is to provide such a vent means whereby when the engine is operating, the fuel tank vapors are vented either directly or via the charcoal canister to the fuel-air intake manifold for the engine and burned therein.
Other objects of this invention will appear in the following description and appended claims, reference being had to the accompanying drawings forming a part of this specification wherein like reference characters designate corresponding parts in the several views.